What is hfr strain

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About Acharya Tankeshwar Articles. Similar statements can be made about each gene, except when it appears at one end or the other of the linkage map. The order in which the genes are transferred is not constant. In two Hfr strains, for example, the his gene is transferred before the gly gene his is closer to O , but, in three strains, the gly gene is transferred before the his gene.

How can we account for these unusual results? Circularity of the E. The orientation in which F is inserted would determine the polarity of the Hfr chromosome , as indicated in Figure a. At one end of the integrated F factor would be the origin, where transfer of the Hfr chromosome begins; the terminus at the other end of F would not be transferred unless all the chromosome had been transferred.

Because the chromosome often breaks before all of it is transferred and because the F terminus is what confers maleness, then only a small fraction of the recipient cells would be converted into male cells.

How, then, might F integration be explained? Campbell then came up with a brilliant extension of that idea. He proposed that, if F, like the chromosome, were circular, then a crossover between the two rings would produce a single larger ring with F inserted Figure Now suppose that F consists of three different regions, as shown in Figure If the bacterial chromosome had several homologous regions that could match up with the pairing region of F, then different Hfr chromosomes could be easily generated by crossovers at these different sites.

Chromosomal and F circularity were wildly implausible concepts initially, inferred solely from the genetic data; confirmation of their physical reality came only a number of years later. The direct-crossover model of integration also was subsequently confirmed. Insertion of the F factor into the E. Hypothetical markers 1 and 2 are shown on F to depict the direction of insertion.

The origin O is the mobilization point where insertion begins; the pairing region is homologous more A frightening ability of pathogenic bacteria was discovered in Japanese hospitals in the s. Bacterial dysentery is caused by bacteria of the genus Shigella.

This bacterium initially proved sensitive to a wide array of antibiotics that were used to control the disease. In the Japanese hospitals, however, Shigella isolated from patients with dysentery proved to be simultaneously resistant to many of these drugs, including penicillin, tetracycline, sulfanilamide, streptomycin, and chloramphenicol.

This multiple-drug-resistance phenotype was inherited as a single genetic package, and it could be transmitted in an infectious manner—not only to other sensitive Shigella strains, but also to other related species of bacteria. This talent is an extraordinarily useful one for the pathogenic bacterium, and its implications for medical science were terrifying.

From the point of view of the geneticist, however, the situation is very interesting. The vector carrying these resistances from one cell to another proved to be a self -replicating element similar to the F factor. In fact, these R factors proved to be just the first of many similar F -like factors to be discovered. These elements, which exist in the plasmid state in the cytoplasm , have been found to carry many different kinds of genes in bacteria.

Table shows some of the characteristics that can be borne by plasmids. The answer is no unless the culture is treated with streptomycin. Otherwise, transferred fragments of DNA in the recipient are lost in the course of cell division.

How is the transfer achieved? We can now summarize the various aspects of the conjugation cycle in E. Hfr cells have F integrated into the bacterial chromosome , not in the cytoplasm. Summary of the various events that take place in the conjugational cycle of E. Because conjugation experiments are usually carried out by mixing from 10 7 to 10 8 cells consisting of prospective donors and recipients, the population will contain various different Hfr cells derived from independent integrations of F into the chromosome at various different sites.

Therefore, when chromosomal markers are transferred by different cells in the population, transfer will start at different points on the chromosome. This results in an approximately equal transfer of markers all around the chromosome, although at a low frequency. Hfr strains are derived from a clone of Hfr cells in which a specific integration of F into the bacterial chromosome has taken place.

Therefore, all the cells in any given Hfr strain have F integrated into the chromosome at exactly the same point. Further, in any given Hfr strain , the markers are transferred from a fixed point in a specific order.

Thus far, we have studied only the process of the transfer of genetic information between individuals in a cross. This transfer is inferred from the existence of recombinants produced from the cross. We now consider some of the special properties of this exchange event.

What we have in fact is a partial diploid , or merozygote. Bacterial genetics is merozygous genetics. Figure a is a diagram of a merozygote. Crossover between exogenote and endogenote in a merozygote.

A single crossover would not be very useful in generating viable recombinants, because the ring is broken to produce a strange, partly diploid linear chromosome Figure b. To keep the ring intact, there must be an even number of crossovers Figure c. The fragment produced in such a crossover is only a partial genome , which is generally lost in subsequent cell growth.

Since Hfr strain has F plasmid or fertility factor it can act as a donor or male bacterium in bacterial conjugation. Some parts of bacterial chromosome or the entire chromosome can also be copied and transferred to the recipient bacterium when Hfr strain is involved is conjugation.

Such Hfr strains are very useful in studying gene linkage and recombination. Hence, molecular biologists and geneticists use Hfr strain of bacteria often E. High-frequency recombination occurs when a recipient bacterium receives three types of DNA after mating with Hfr strain through bacterial conjugation.

Due to this reason, such bacteria are named as Hfr strains. F plasmids can integrate into bacterial chromosome and disintegrate back from the host chromosome. During disintegration, F plasmid can pick some genes near it from the host chromosome.

F plasmids contain a fertility factor or F factor which is essential for bacterial conjugation. These bacteria are able to transfer their F plasmid into bacteria which lack F plasmids. Once these F plasmids enter into recipient bacterium, it can exist independently or it can integrate with bacterial chromosome.

A high-frequency recombination cell Hfr cell also called an Hfr strain is a bacterium with a conjugative plasmid for example, the F-factor integrated into its chromosomal DNA.

The integration of the plasmid into the cell's chromosome is through homologous recombination. They are designated as F - simply because they do not have F plasmid. Bacterial conjugation is often incorrectly regarded as the bacterial equivalent of sexual reproduction or mating. It is not actually sexual, as it does not involve the fusing of gametes and the creation of a zygote. It is merely the transfer of genetic information from a donor cell to a recipient.

Experiment demonstrating that physical contact between bacterial cells is needed for genetic recombination to take place. A suspension of a bacterial strain unable to synthesize certain nutrients is placed in one arm of a U - tube.

During conjugation , one bacterium serves as the donor of the genetic material, and the other serves as the recipient. The donor bacterium carries a DNA sequence called the fertility factor, or F-factor. For instance, in many cases, conjugation serves to transfer plasmids that carry antibiotic resistance genes. Also called F factor , fertility factor. A technique used to map bacterial genes by determining the sequence in which donor genes enter recipient cells.